The present invention is directed to bicycle transmissions and, more particularly, to a bicycle hub transmission using a planet gear mechanism for switching gear ratios.
Small bicycles (referred to as "BMX bicycles") having 20-inch wheels are used in off-road motocross racing. A fast start is an important element of BMX motocross racing. Thus, downshifting to a slightly lighter gear ratio than during normal riding is recommended during startup in order to achieve higher startup acceleration. For this reason, BMX bicycles used in motocross racing are equipped with an external shifter comprising a rear derailleur and a hub cog having two cross-ratio sprockets, wherein a shifting lever is linked by a cable to the rear derailleur. Installing an external shifter, however, makes it necessary for the rider to control this shifter by operating a shifting lever. Since BMX motocross racing involves navigating a course having a succession of small bumps and curves, the rider is busy steering and does not have much time for shifting. Consequently, it is very difficult to shift gears during a race.
To overcome this problem, the rear wheel may be equipped with an internal shifter hub, and the gears may be shifted automatically by changing the speed of the internal shifter hub in accordance with the bicycle speed. Such an internal shifter hub comprises a hub axle, a driver capable of rotating about the hub axle, a hub shell, a planet gear mechanism for changing the speed of rotation of the driver and transmitting the result to the hub shell, a clutch mechanism for transmitting the output of the planet gear mechanism to the hub shell or stopping such transmission, and a clutch-switching mechanism for switching the clutch mechanism by centrifugal force, for example.
Bearing components are provided between one end of the driver and the hub axle, between one end of the hub shell and the other end of the driver, and between the other end of the hub shell and the hub axle, thus allowing the driver and the hub shell to rotate about the hub axle. The planet gear mechanism comprises an inner-tooth gear, a sun gear, a plurality of planet gears for meshing with the inner-tooth gear and the sun gear, and a planet gear carrier for supporting the plurality of planet gears. The clutch-switching mechanism switches the clutch mechanism between an engaged state and a disengaged state using centrifugal force created by the rotation of the hub shell.
When the gear ratio of an internal shifter hub thus constructed is lowered slightly (that is, when the rotation of the hub shell remains mostly unchanged in relation to the rotation of the driver), the number of teeth of the inner-tooth gear must be fairly high in relation to the number of teeth of the sun gear. Consequently, the gap between the inner-tooth gear and the sun gear is increased, as is the outside diameter of the planet gears. The outside diameters of the planet gears are sometimes also increased when the gear ratio is raised. When, for example, a planet gear consists of two gear elements (a large element and a small element), the sun gear is caused to mesh with the small gear element, and the inner-tooth gear is caused to mesh with the large gear element. In either case, the outside diameter of the driver often must be increased when, for example, an inner-tooth gear is formed on the inner peripheral surface of the driver, and a downshift is performed by inputting motive force from the inner-tooth gear and providing an output from a carrier via the planet gears. Increasing the outside diameter of the driver results in a corresponding increase in the outside diameter of at least one of the bearing components provided at the two ends of the driver. This, in turn, results in an overall increased outside diameter of the internal shifter hub.